In many areas of medical diagnosis and therapy, it is desired to selectively deliver an agent, such as a therapeutic agent (a drug) or a diagnostic (e.g. imaging) agent, to a specific site, or a confined region, in the body of a subject such as a patient.
Active targeting of an organ or a tissue is achieved by the direct or indirect conjugation of the desired active moieties (e.g. a contrast enhancing agent or a cytotoxic compound) to a targeting construct, which binds to cell surfaces or promotes cellular uptake at or near the target site of interest. The targeting moieties used to target such agents are typically constructs that have affinity for cell surface targets (e.g., membrane receptors), structural proteins (e.g., amyloid plaques), or intracellular targets (e.g., RNA, DNA, enzymes, cell signalling pathways). These moieties can be antibodies (fragments), proteins, aptamers, oligopeptides, oligonucleotides, oligosaccharides, as well as peptides, peptoids and organic drug compounds known to accumulate at a particular disease or malfunction. Alternatively, a contrast/therapeutic agent may target a metabolic pathway, which is upregulated during a disease (like infection or cancer) such as DNA, protein, and membrane synthesis and carbohydrate uptake. In diseased tissues, abovementioned markers can discriminate diseased cells from healthy tissue and offer unique possibilities for early detection, specific diagnosis and (targeted) therapy.
An important criterion for successful molecular imaging/therapy agents in general and nuclear imaging/therapy agents in particular is that they exhibit a high target uptake while showing a rapid clearance (through renal and/or hepatobiliary systems) from non-target tissue and from the blood. However, this is often problematic: for example, imaging studies in humans have shown that the maximum concentration of a radio labeled antibody at the tumor site is attainable within 24 h but several more days are required before the concentration of the labeled antibody in circulation decreases to levels low enough for successful imaging to take place.
These problems (especially for nuclear imaging and therapy) with slow or insufficient accumulation in target tissue and slow clearance from non-target areas have lead to the application of pre-targeting approaches.
Pretargeting refers to a step in a targeting method, wherein a primary target (e.g. a cell surface) is provided with a Pre-targeting Probe. The latter comprises a secondary target, which will eventually be targeted by a further probe (the Effector Probe) equipped with a secondary targeting moiety.
Thus, in pre-targeting, a Pre-targeting Probe is bound to a primary target. The Pre-targeting Probe also carries secondary targets (sometimes also referred as “tags”), that facilitate specific conjugation to a diagnostic (imaging) and/or therapeutic agent, the Effector Probe. After the construct forming the Pre-targeting Probe has localized at the target site (taking time, e.g. 24 h), excess of the Pre-Targeting Probe has to be removed from the blood.
As natural clearance frequently is not sufficient, it is desired to use a clearing agent. Clearing agents work to remove compounds from circulation by directing them to a specific organ or tissue. Thus, in general, a clearing agent is an agent capable of binding, complexing or otherwise associating with an administered moiety (e.g., targeting moiety-ligand, targeting moiety-anti-ligand or anti-ligand alone) present in the recipient's circulation, thereby facilitating circulating moiety clearance from the recipient's body, removal from blood circulation, or inactivation thereof in circulation. The clearing agent is preferably characterized by physical properties, such as size, charge, configuration or a combination thereof, that reduce clearing agent access to the population of target cells recognized by a targeting moiety used in the same treatment protocol as the clearing agent. Typically, clearing agents bind to the pre-targeting antibody. Alternatively, it would be desired to provide clearing agents that by mere rapid clearance from circulation prior to being able to access a target cell. To this end, it is desired to provide clearing agents capable of reacting fast (exhibiting a high reaction rate with the biomolecule to be cleared).
In US 2003/0129191 clearing agents are disclosed that incorporate ligand derivatives or anti-ligand derivatives. Herein, essentially, such derivatives exhibit a lower affinity for the complementary ligand/anti-ligand pair member than the native form of the compound. Thus, e.g., where a biotin-avidin or biotin-streptavidin ligand/anti-ligand pair is employed, the clearing agents incorporate either a biotin derivative exhibiting a lower affinity for avidin or streptavidin than biotin or an avidin or a streptavidin derivative exhibiting a lower affinity for biotin than avidin or streptavidin.
Rossin et al., Angew. Chem. Int., Ed 2010, 49, p. 3375-3378 relates to tumor pretargeting by using the inverse-electron-demand Diels-Alder reaction.
WO 2010/119382 A1 and WO 2010/119389 A2 refer to pretargeting methods involving the use of [4+2] inverse-electron-demand (retro) Diels-Alder chemistry in providing the coupling between a pretargeting probe and an effector probe.
U.S. Pat. No. 7,052,872 B1 relates to a bi-specific antibody or antibody fragment having at least one arm that specifically binds a targeted tissue and at least one arm that specifically binds a targetable conjugate.
U.S. Pat. No. 5,965,131 A refers to a pretargeting method for delivering diagnostic or therapeutic agents to a target site using a clearing agent that binds to the target-binding site of the targeting species, wherein anti-idiotype antibodies and antibody fragments are preferred clearing agents.
Moreover, the current pretargeting systems are hampered by factors associated with their biological nature. Biotin is an endogenous molecule and its conjugates can be cleaved by the serum enzyme biotinidase. When antisense pre-targeting is used, the oligonucleotides can be subject to attack by RNAse and DNAse. Proteins and peptides are also subject to natural decomposition pathways. These interactions can be further impaired by their non-covalent and dynamic nature and limited on-target residence time. Also, endogenous biotin competes with biotin conjugates for streptavidin binding. Finally, streptavidin is highly immunogenic.
Further, the imaging of targets is sometimes hampered by circulating fractions of said target, that capture the imaging agents before they can reach the target at the desired locus. It is desired that a system be provided by which such circulating targets can be removed prior to administering the imaging agent. Also in other circumstances, e.g. as a therapy, it can be desired to remove biomolecules from circulation. E.g., if therapy is directed to interaction with an enzyme such as MMP-2 (matrix metalloproteinase-2) that occurs in the extracellular matrix of certain tumors, but also in serum, a clearance concept will be desired. Another example is CEA (carcinoembryonic antigen) which is a target for (pre)targeting of radioimmunotherapy, but it also occurs in serum and will therein capture radioactivity.
Other examples are antibody conjugates used for Antibody Directed Enzyme Prodrug Therapy (ADEPT), where it would be beneficial to remove any antibody conjugate from circulation prior to administering the prodrug. Other therapeutic agents that could benefit from a clearing approach are antibody-drug conjugates. These conjugates circulate for a long time but comprise highly toxic drugs, requiring the conjugate linker to be highly stable. Using a clearing agent would allow to remove any free antibody drug conjugate from circulation after sufficient amount has bound to the tumor.
It is desired that a clearing concept be found that is versatile, that allows providing clearing agents that rapidly bind to administered moieties to be cleared from circulation, and that can be used in pre-targeting approaches.